Throttling diesel nozzles have recently come into widespread use in indirect injection automotive and light-duty diesel truck engines, i.e., compression ignition engines in which the fuel is injected into and ignited in a prechamber or swirl chamber. In this way, the flame front proceeds from the prechamber into the larger compression chamber where the combustion is completed. Engines designed in this manner allow for quieter and smoother operation. The FIGURE of the Drawing illustrates the geometry of the typical throttling diesel nozzle (often referred to as the "pintle nozzle").
Unfortunately, the advent of such engines has given rise to a new problem, that of excessive coking on the critical surfaces of the injectors that inject fuel into the prechamber or swirl chamber of the engine. In particular and with reference to the FIGURE, the carbon tends to fill in all of the available corners and surfaces of the obturator 10 and the form 12 until a smooth profile is achieved. The carbon also tends to block the drilled orifice 14 in the injector body 16 and fill up to the seat 18. In severe cases, carbon builds up on the form 12 and the obturator 10 to such an extent that it interfers with the spray pattern of the fuel issuing from around the perimeter of orifice 14. Such carbon build up or coking often results in such undesirable consequences as delayed fuel ignition, decreased rate of fuel injection, increased rate of combustion chamber pressure rise, increased engine noise, and can also result in an excessive increase in emissions from the engine of unburned hydrocarbons.
While the composition of the low cetane number fuel is believed to be a major contributing factor to the coking problem, it is not the only relevant factor. Thermal and oxidative stability (lacquering tendencies), fuel aromaticity, and such fuel characteristics as viscosity, surface tension and relative density have also been indicated to play a role in the coking problem.
An important contribution to the art would be a fuel composition which has enhanced resistance to coking tendencies when employed in the operation of indirect injection diesel engines.